On Windows, an error running the debugger typically leaves a process hanging around in the working directory. When Dexter exits, it can't then delete the working directory and produces an exception, masking the problem in the debugger. (This can be worked around by specifying --save-temps). Rather than hard-erroring, print a warning when we can't delete the working directory instead.
It'd be much better to improve our error handling, and make the WorkingDirectory class aware that something's wrong when it enters exit. However, this is something that's going to mask genuine errors and make everyones lives harder right now, so I think this non-ideal fix is important to get in first.